Shrinking Salmon

This graph shows the decreasing weight of pink salmon in two rivers in British Columbia, the Bella Coola and the Upper Johnston Straight. From 1950 to 1974, the size of pink salmon decreased dramatically because humans, removing the largest fish for their dinners, act as a selective pressure -- selecting for smaller, slower-growing fish. There are two lines for each river, representing salmon born in odd and even years. Because of the two-year life cycle of pink salmon, these two populations are genetically distinct and have different average weights.

It may sound like an exaggerated memory, but our grandfathers' stories of bigger fish on fishing trips years ago just may be true. Some fish today are smaller than in the past, and overfishing may be why.

Part of the reason seems simple. So many fishermen are catching fish these days, few fish have a chance to live long enough to grow very big. But there is also evidence that some fish are growing more slowly than their ancestors -- staying small enough to escape fishing nets designed to catch larger fish. The little fish, artificially selected by escaping the nets, then become the parent stock of the next generation. Those smaller fish repeat the cycle until the average size of that species of fish becomes measurably reduced.

Pink salmon may be the clearest example of this phenomenon. This species, Oncorhyncus gorbuscha, is one of five species of salmon, and it has a distinctive and predictable lifestyle. Pink salmon follow the pattern typical of other salmon species at first, hatching from eggs laid in freshwater streams, and drifting downstream to the sea almost as soon as they emerge. But unlike other salmon, which have more flexible life cycles, pink salmon return like clockwork after two years to mate and spawn and die in the stream where they were born. This punctuality means that fish born in odd years return in the next odd year and mate with other odd-year fish, while even-year fish do the same in even years. Thus these two populations are genetically distinct, and have different average sizes. And because all returning salmon are the same age, it's relatively easy to see the process of evolution at work.

Decades of overfishing resulted in 80 percent of the spawning stock winding up on dinner plates, and t the average size of a pink salmon coming to rivers to spawn has declined 30 percent in the last 40 years. This trend can't be explained away by fishermen catching the oldest fish, since these salmon always return to spawn at the same age. What the declining size shows instead is the evolution of pink salmon with slower growth rates, and a smaller size at maturity. Big fish produce more eggs than small ones and contribute more offspring to the next generation, so in the absence of fishing pressure, natural selection tends to favor the larger fish. These days, however, the larger fish are more likely to end up in a fishing net than to make it back to the stream where they were born in order to spawn. Human fishing pressure has tipped the evolutionary balance in favor of the smaller salmon.